Usually a few kt's are added to the Vref as I understand it. (+5)?
It can be a lot more than a few knots - how about plus 20 knots above the Vref?
Boeing recommend add half the steady headwind component and all of the gust factor. While Boeing also say the half the steady HW component should be bled off approaching touch-down, in reality that rarely occurs with the result that a good proportion of touch-downs occur faster than they should. No problem with a long runway but a flight safety problem on a short wet runway especially if the additives cause an unintentional float.